(1) Field of the Invention
The present invention relates to acoustic line arrays and more particularly to a method of determining optimum hydrophone element spacing for a low frequency, log-periodic acoustic line array which transmits and receives over a broad range of low frequencies while retaining a beam pattern having approximately the same directional characteristics over a decade range of frequencies.
(2) Description of the Prior Art
Antennas with variable-type apertures constituted a major breakthrough in the field of wideband electromagnetic radiators primarily due to the invention of frequency independent and log-periodic structures used in both the transmit and receive modes with the object being to retain over some frequency band the same directional characteristics of the beam. Frequency independent antenna concepts have been previously applied to underwater sound systems operating in the ultrasonic frequency range. To date, however, no comparable method exists which adequately addresses element spacing for the lower end of the audio frequency range.
In one instance, Hixson and Au (E. L. Hixson and K. T. Au, University of Texas, Acoustics Research Laboratory, Technical Memorandum No. 19 of May 1, 1970) proposed a wideband constant beamwidth acoustic array utilizing quasi-logarithmic element spacing covering the 300 to 3000 Hz range. The method they used was to superimpose, on an array of given length and spacing, successive arrays of half the size and spacing while removing redundant elements to approximate a logarithmic element arrangement. A method predicting exact element location does not exist at the present time.
For a constant aperture the beamwidth and associated directivity index of an acoustic transducer varies with frequency and this is also true for transducer arrays. This effect limits the performance of wideband acoustic systems because a change or loss of directivity over the intended operational frequency will adversely affect detection capabilities. Uniformly spaced linear arrays are subject to these limitations (i.e., they are basically narrow band devices). Recent advances in transducer array design have utilized several shading techniques to reduce side lobe levels for optimum main beam characteristics. However, the problem of optimizing the broadband qualities of a low frequency omnidirectional transducer has not been fully addressed to date.
The present invention describes a method which shows how the log-periodic approach can be extended to the low end of the audio frequency regime (20 to 3000 Hz) in the design of acoustic line arrays to yield quasi-constant directionality over frequencies in excess of one decade for Broadside as well as End-Fire Arrays. Exact element location is predicted by criteria derived from the design of log-periodic dipole antennas. It should be noted that although this specification discloses the retention of acceptable dipole and cardioid patterns in selected bandwidths, the method can be applied to any array size and frequency range as long as the physical spacing of the elements remains feasible.